Chest wall mechanics: effects of acute and chronic lung disease

Data from the literature show that lung tissue properties affect the chest wall compliance, Ccw, which is the change in lung volume, Vl, with respect to the pleural pressure, Ppl. to analyze the difference between acute and chronic lung tissue changes, we used a mathematical model that describes the static, nonlinear mechanics of the ventilatory system in terms of its major elements: rib cage; abdomen; diaphragm and lung. With this model we derived the relationship between chest wall, rib-cage and diaphragm compliances. Although the Vl-Ppl relation is independent of lung mechanics, the volume operating point (FRC) of the ventilatory system depends on lung tissue properties. This accounts for the effect of acute lung abnormalities. In the presence of chronic lung abnormalities, the properties of the rib-cage are changed which shifts the entire Vl-Ppl curve. In general, valid comparisons of (extra-pulmonary) chest wall mechanics can only be made using the entire Vl-Ppl relation, or at least a sufficiently large part of the relation about FRC. Differentiation of the rib-cage and diaphragm mechanics requires additional measurements of the rib-cage A-P distance and the relative position of the diaphragm.     

Frequency-dependent effects of hypercapnia on respiratory mechanics of cats

The effect of increasing arterial partial pressure of CO2 (PaCO2) on respiratory mechanics was investigated in six anesthetized, paralyzed cats ventilated by constant-flow inflation. Respiratory mechanics were studied after end-inspiratory occlusions. Zero frequency resistance (Rmax), infinite frequency resistance (Rmin), and static elastance (Est) were calculated for the respiratory system, lung, and chest wall. Alveolar ventilation was manipulated by the addition of dead space to achieve a range of PaCO2 values of 29.3-87.3 mmHg. Cats did not become hypoxic during the experiment. Under control conditions marked frequency dependence in Rmax, Rmin, and Est of the respiratory system, lungs, and chest wall was demonstrated. The chest wall contributed 50% of the total resistance of the respiratory system. With increasing PaCO2 the only resistance observed to increase was Rmax of the lung (P less than 0.01). There were also no changes in the static elastic properties of either the lungs or the chest wall. These results suggest that hypercapnia increases resistance by changes in the lung periphery and not in the conducting airways.     

Chest wall mechanics in sustained microgravity

We assessed theeffects of sustained weightlessness on chest wall mechanics in fiveastronauts who were studied before, during, and after the 10-daySpacelab D-2 mission (n = 3)and the 180-day Euromir-95 mission (n = 2). We measured flow and pressure at the mouth and rib cage andabdominal volumes during resting breathing and during a relaxationmaneuver from midinspiratory capacity to functional residual capacity.Microgravity produced marked and consistent changes () in thecontribution of the abdomen to tidal volume [Vab/(Vab + Vrc), where Vab is abdominal volume and Vrc is rib cagevolume], which increased from 30.7 ± 3.5 (SE)% at1 G head-to-foot acceleration to 58.3 ± 5.7% at 0 G head-to-foot acceleration (P < 0.005). Values ofVab/(Vab + Vrc) did not change significantly during the 180 days of the Euromir mission, but in the two subjects Vab/(Vab + Vrc) was greater on postflight day1 than on subsequent postflight days or preflight. Inthe two subjects who produced satisfactory relaxation maneuvers, the slope of the Konno-Mead plot decreased in microgravity; this decrease was entirely accounted for by an increase in abdominal compliance because rib cage compliance did not change. These alterations aresimilar to those previously reported during short periods ofweightlessness inside aircrafts flying parabolic trajectories. They arealso qualitatively similar to those observed on going from upright tosupine posture; however, in contrast to microgravity, such posturalchange reduces rib cage compliance.

     

Effects of volume history and vagotomy on pulmonary and chest wall mechanics in cats

Using the technique of rapid airway occlusion during constant-flow inflation, we studied the effects of inflation volume, different baseline tidal volumes (10, 20, and 30 ml/kg), and vagotomy on the resistive and elastic properties of the lungs and chest wall in six anesthetized tracheotomized paralyzed mechanically ventilated cats. Before vagotomy, airway resistance decreased significantly with increasing inflation volume at all baseline tidal volumes. At any given inflation volume, airway resistance decreased with increasing baseline tidal volume. After vagotomy, airway resistance decreased markedly and was no longer affected by baseline tidal volume. Prevagotomy, pulmonary tissue resistance increased progressively with increasing lung volume and was not affected by baseline tidal volume. Pulmonary tissue resistance decreased postvagotomy. Chest wall tissue resistance increased during lung inflation but was not affected by either baseline tidal volume or vagotomy. The static volume-pressure relationships of the lungs and chest wall were not affected by either baseline tidal volume or vagotomy. The data were interpreted in terms of a linear viscoelastic model of the respiratory system (J. Appl. Physiol. 67: 2276-2285, 1989).     

Chest wall mechanics during artificial ventilation.

Chest wall mechanics were studied in six healthy volunteers before and during anesthesia prior to surgery. The intratracheal, esophageal, and intragastric pressures were measured concurrently. Gas flow was measured by pneumotachography and gas volume was obtained from it by electrical integration. Rib cage and abdomen movements were registered with magnetometers, these being calibrated by "isovolume" maneuvers. During spontaneous breathing in the conscious state, rib cage volume displacement corresponded to 40% of the tidal volume. During anesthesia and artificial ventilation, this rose to 72% of the tidal volume. The relative contributions of rib cage and abdomen displacements were not influenced by a change in tidal volume. Compliance was higher with a larger tidal volume, a finding which could be due to a curved pressure-volume relationship of the overall chest wall.     

Flow and volume dependence of pulmonary mechanics in anesthetized cats

The effects of inspiratory flow rate and inflation volume on pulmonary mechanics were investigated in six anesthetized-paralyzed cats ventilated by constant-flow inflation. Pulmonary mechanics were assessed using the technique of rapid airway occlusion during constant-flow inflation which allows measurement of the intrinsic pulmonary resistance (RLmin) and of the overall "pulmonary flow resistance" (RLmax), which includes the additional pulmonary pressure losses due to time constant inequalities within the lung and/or stress adaptation. We observed that, at fixed inflation volume, 1) RLmin fitted Rohrer's equation, 2) RLmax was higher at low than intermediate flows, and 3) RLmax-RLmin decreased progressively with increasing flow. At fixed flow, RLmax increased, whereas RLmin decreased with increasing volume. We conclude that during eupneic breathing in cats, the pulmonary flow resistance as conventionally measured includes a significant component reflecting stress adaptation.     

Analysis of behavior of the respiratory system in ARDS patients: effects of flow, volume, and time

The effects of inspiratory flow (V) and inflation volume (delta V) on the mechanical properties of the respiratory system in eight ARDS patients were investigated using the technique of rapid airway occlusion during constant-flow inflation. We measured interrupter resistance (Rint,rs), which in humans represents airway resistance, the additional resistance (delta Rrs) due to viscoelastic pressure dissipations and time constant inequalities, and static (Est,rs) and dynamic (Edyn,rs) elastance. The results were compared with a previous study on 16 normal anesthetized paralyzed humans (D'Angelo et al. J. Appl. Physiol. 67: 2556-2564, 1989). We observed that 1) resistance and elastance were higher in ARDS patients; 2) with increasing V, Rint,rs and Est,rs did not change, delta Rrs decreased progressively, and Edyn,rs increased progressively; 3) with increasing delta V, Rint,rs decreased slightly, delta Rrs increased progressively, and Est,rs and Edyn,rs showed an initial decrease followed by a secondary increase noted only in the ARDS patients. The above findings could be explained in terms of a model incorporating a standard resistance in parallel with a standard elastance and a series spring-and-dashpot body that represents the stress adaptation units within the tissues of the respiratory system.     

Expiratory flow pattern and respiratory mechanics

During resting breathing, expiration is characterized by the narrowing of the vocal folds which, by increasing the expiratory resistance, raises mean lung volume and airway pressure. This is even more pronounced in the neonatal period, during which expirations with short complete airway closure are commonly occurring. We asked to which extent differences in expiratory flow pattern may modify the inspiratory impedance of the respiratory system. To this aim, newborn puppies, piglets, and adult rats were anesthetized, paralyzed, and ventilated with different expiratory patterns, (a) no expiratory load, (b) expiratory resistive load, and (c) end-inspiratory pause. The stroke volume of the ventilator and inspiratory and expiratory times were maintained constant, and the loads were adjusted in such a way that inflation always started from the resting volume of the respiratory system. After 1 min of each ventilatory pattern, mean inspiratory impedance and compliance of lung and respiratory system were measured. The values were unchanged or minimally altered by changing the type of ventilation. We conclude that the expiratory laryngeal loading is not primarily aimed to decrease the work of breathing. It is conceivable that the expiratory pattern is oriented to increase and control mean airway pressure in the regulation of pulmonary fluid reabsorption, distribution of ventilation, and diffusion of gases.     

Interrupter technique for measurement of respiratory mechanics in anesthetized cats

In six spontaneously breathing anesthetized cats (pentobarbital sodium, 35 mg/kg ip), airflow, changes in lung volume, and tracheal and esophageal pressures were measured. Airflow was interrupted by brief airway occlusions during relaxed expirations (elicited via the Breuer-Hering inflation reflex) and throughout spontaneous breaths. A plateau in tracheal pressure occurred throughout relaxed expirations and the latter part of spontaneous expirations indicating respiratory muscle relaxation. Measurement of tracheal pressure, immediately preceding airflow, and corresponding volume enabled determination of respiratory system elastance and flow resistance. These were partitioned into lung and chest wall components using esophageal pressure. Respiratory system elastance was constant over the tidal volume range, divided approximately equally between the lung and chest wall. While the passive pressure-flow relationship for the respiratory system was linear, those for the lung and chest wall were curvilinear. Volume dependence of chest wall flow resistance was demonstrated. During inspiratory interruptions, tracheal pressure increased progressively; initial tracheal pressure was estimated by backward extrapolation. Inspiratory flow resistance of the lung and total respiratory system were constant. Force-velocity properties of the contracting inspiratory muscles contributed little to overall active resistance.     

Effects of longitudinal laparotomy on respiratory system, lung, and chest wall mechanics.

In six sedated, anesthetized, paralyzed, and mechanically ventilated guinea pigs, total respiratory system (RT,rs), lung, and chest wall resistances and respiratory system (Est,rs), lung, and chest wall (Est,w) elastances were determined before and after longitudinal laparotomy. Furthermore the resistances were also split into their initial and difference components, with the former reflecting the Newtonian resistances and the latter representing the viscoelastic/inhomogeneous pressure dissipations in the system. For such purpose the end-inflation occlusion during constant inspiratory flow method was used. During laparotomy, a statistically significant increase in respiratory system difference resistance (from 0.086 to 0.101 cmH2O.ml-1.s) significantly augmented RT,rs (from 0.157 to 0.167 cmH2O.ml-1.s). The former was entirely secondary to a significant increase in chest wall difference resistance (0.019 to 0.034 cmH2O.ml-1.s), which naturally raised chest wall total resistance (from 0.030 to 0.047 cmH2O.ml-1.s). Est,rs and Est,w also increased (14.7 and 13.1%, respectively) after abdominal incision. It can be concluded that the midline xiphipubic laparotomy accompanied by the bilateral ventrodorsal infracostal incision increases RT,rs as a consequence of augmented chest wall difference resistance and Est,rs as a result of higher Est,w.     

High-frequency oscillatory ventilation in neonatal RDS: initial volume optimization and respiratory mechanics

To determine whetherinitial lung volume optimization influences respiratory mechanics,which could indicate the achievement of optimal volume, we studied 17 premature infants with respiratory distress syndrome (RDS) assisted byhigh-frequency oscillatory ventilation. The continuous distendingpressure (CDP) was increased stepwise from 6-8 cmH₂Oup to optimal CDP (OCDP), i.e., that allowing good oxygenation with thelowest inspired O₂ fraction. Respiratory systemcompliance (Crs) and resistance were concomitantlymeasured. Mean OCDP was 16.5 ± 1.2 cmH₂O. InspiredO₂ fraction could be reduced from an initial level of 0.73 ± 0.17 to 0.33 ± 0.07. However, Crs (0.45 ± 0.14ml · cmH₂O¹ · kg¹at starting CDP point) remained unchanged through lung volume optimization but appeared inversely related to OCDP. Similarly, respiratory system resistance was not affected. We conclude that thereis a marked dissociation between oxygenation improvement and Crsprofile during the initial phase of lung recruitment by earlyhigh-frequency oscillatory ventilation in infants with RDS. Thusoptimal lung volume cannot be defined by serial Crs measurement. At themost, low initial Crs suggests that higher CDP will be needed.

     

Chest wall distortion and regional lung volume distribution in erect humans.

Using 133Xe measured the regional distribution of FRC and of boluses administered at FRC in seated subjects during relaxation, lateral compression of the lower rib cage, and contraction of the inspiratory muscles so that mouth pressure was 50 cmH2O subatmospheric. Lateral compression increased apex-to-base differences of volume and bolus distribution, suggesting an increase of the apex-to-base gradient of pleural surface pressure. Changes in rib cage shape were measured with magnetometers and were qualitatively similar to those associated with increases in apex-to-base difference of pleural surface pressure in animals. Inspiratory effort decreased apex-to-base difference in volume and induced a similar trend in bolus distribution. Though changes in the rib cage shape were directionally similar, they were much smaller than those associated with decreased pleural surface pressure gradients in animals, and the changes in regional volume we observed were more likely due to forces generated by diaphragmatic contraction. These results were compatible with the apex-to-base gradient of pleural pressure being strongly influenced by shape adaptation between lung and chest wall.